Pressure is one of the most widely measured physical parameters, having significant importance in widely varying fields in industry, technology and science. Systems for measuring pressures at high operating temperatures have particular needs.
Conventional high temperature pressure-sensing transducers are often limited in operation, as the overall pressure sensing system, which includes associated electronics, is limited to relatively low operating temperatures.
Conventional electronic conditioning units used with pressure transducers are typically limited to operating temperatures below around +125° C. (+257° F.). While carefully designed circuits may be capable of operating up to around +175° C. (+347° F.), owing to very complicated process and design optimization with significant performance sacrifices, systems that operate at even higher temperatures are desired for certain applications.
It is desirable to provide a pressure sensing system (transducer) suitable for use at high operating temperatures, greater than +125° C. (+257° F.) and even +175° C. (+347° F.). For added functionality and versatility, the transducer should incorporate a conditioning unit to generate a high level output, properly normalized and compensated. Also the conditioning unit should incorporate means to reduce or cancel the effects of the variations of the input voltage, allowing the operation of the transducer over a wide range of input voltages.
Temperature limitations of conventional pressure sensing systems result from leakage currents between individual components of an integrated circuit—e.g., between transistors and resistors, and the bulk substrate. Circuit components are typically isolated from one other and the substrate by reverse polarized p-n junctions. While the reverse bias current of these junctions is typically low at room temperature, this current tends to double about every 6° C. As a result, above certain temperatures, the leakage currents become too high and render the circuit unsuitable. This temperature limit is typically around +175° C. (+347° F.).
The operating temperature of the transducer can be further increased by the use of Complementary Metal-Oxide Semiconductor (CMOS) devices for the signal conditioning unit. CMOS devices are majority carrier type semiconductors, which enables them to operate at higher temperature than bipolar linear circuits, based on p-n junctions and minority carriers.